Carbon filaments

ABSTRACT

CARBON FILAMENTS ARE FORMED BY GRAPHITIZING A CARBON CONTAINING FILAMENT IN AN ATMOSPHERE OF INERT GAS AND VAPORS OF GROUPS I, II, IV AND VI METALS.

Sept. 19, 1972 Filed Dec. 2. 1969 C. G. EVANS CARBON FILAMENTS 2 Sheets-Sheet 1 2 Sheets-Sheet 2 Filed Dec. 2. 1969 mozoumm Q2; 8 V V V V 9 c z z mwmm u ZVQQmm United States Patent 11.3. Cl. 117-225 19 Claims ABSTRACT OF THE DISCLOSURE Carbon filaments are formed by graphitizing a carbon containing filament in an atmosphere of inert gas and vapors of Groups I, II, IV and VI metals.

This invention relates to the manufacture of graphitized carbon filaments from carbon-containing filaments.

The carbon-containing filament starting material as used herein is a regenerated cellulosic, modacrylic or acrylic, e.g. polyacrylonitrile filament, and includes monofilaments or bundles of fibres and yarns, braids or fabrics, or papers made from monofilaments, fibre bundles or yarns.

This invention provides a method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heat treatment to graphitize said carbon-containing lament in a furnace containing an atmosphere of an inert gas metal adsorbable by said carboncontaining filament and vaporizable at graphitizing temperatures. The heat treatment may be effected by passing an electric current through the filament and subjecting it to a voltage sufiicient to cause the graphitization thereof.

In cases of graphitization for high strength, metals with boiling points below 2,000 C. should be used and in cases of graphitization for high modulus, metals with boiling points below 3,000 C. should be used. It should be noted that, even metals with very high boiling points may be used where they have high vapor pressures below their boiling points. Reduced furnace pressures may be used to assist vaporization. The technique is analogous to metal deposition by sputtering.

The invention also covers a method of producing a carbon filament comprising the steps of heating a polyacrylonitrile filament to a temperature of from 230 C. to 330 C. for a period sufficient to turn the polymer black; raising the temperature in a furnace containing an inert atmosphere and an electronegative metal with a vaporization temperature below 2,000" O, preferably below 1,500 C., to about 600 C., for a period sufficient to cause the filament to become a resistance heater; and thereafter, raising the temperature of the filament to the graphitizing range, e.g. l,0003,000 C. by the passage of an electric current through the filament for a sufficient time to promote the desired degree of graphitization.

Desirably the filament is held under tension while being subjected to heating.

One of the problems we have tried to overcome is that until the filament is 90% carbonized, although it will conduct electricity, sparking takes place at the contacts, leading to erosion and an unsatisfactory product. It was to overcome this problem of sparking that we used zinc, feeling that zinc would be absorbed into the spacings between the molecules, bridging the gap and eliminating sparking that way, and also bridging the gap between the contacts and the filaments and so eliminate sparking, and this does in fact appear to happen.

However, zinc does have other actions, some proven, some conjecture. Firstly, the zinc vaporizes at a relatively low temperature and any other metals used for this purpose would need to vaporize before 1,000" C. Zinc seems to be present both in the partially carbonized fibre and the fully carbonised fibre. Zinc has an afiinity for oxygen and thus absorbs oxygen in the vapor form and eliminates oxygen etching of the fibre, which is a serious problem. Zinc eliminates conductivity problems due to sparking before the temperature reaches 1,000 C. which is the twilight zone, where the fibre is only partially carbonized. It appears to be soluble in carbon from the vapor phase and it appears that its presence in the fibre facilitates crystal growth. This may be by keeping sufficient spacing between the molecules to allow movement to take place, so that they can line themselves up with each other in the crystallizing stage, while at the same time bridging the gap sufiiciently so that Van der Waal forces are able to pull the crystals together. On the other hand, it may curiously be acting as a cement between the crystals, much as small additions to iron act to improve the crystal structure of steel, e.g. cobalt, nickel, molybdenum and carbon.

We believe therefore, that the best results are obtained with a metal which vaporizes at a temperature below 1,000 C. A metal which is readily adsorbed or absorbed by the fibre from the vapor phase gives improved results. Our studies suggest that electro-negative metals have this particular property. In other words, the structure of the metal should be such that it helps to tie the graphite crystals together. Zinc appears to have all these properties and is still present to the extent of 0.1% when the fibre has been up to 3,000 C., so that it is apparently very strongly held onto the filament.

Other metals believed to be capable of meeting these requirements include calcium, cadmium and magnesium. Magnesium is diificult to use in the vapor phase. Generally, any electro-negative metal vaporizing below 1,000- C. which can be adsorbed or absorbed from the vapor phase into carbon will have a particularly good efiect on carbon crystal growth. Zinc is the most practical and readily available.

In summary, therefore, we believe all the alkali metals to be suitable i.e. Li, Na, K, Rb and Cs, certain Group II metals i.e. Mg, Ca, Ba, Zn and Cd, lead in Group IV and tellurium in Group VI of Mendeleefs Periodic Table. Of these calcium, cadmium and zinc, particularly zinc, are preferred.

An electric potential may be maintained along or across the heated length of filament during the heating process. The preheating may be effected by external heating which is terminated once the heating by passage of electric current through the filament is sufficient to supply the heat required.

The degree of tension depends on the particular characteristics which it is desired to achieve in the final product. In general high modulus and high strength result from high tension, low modulus and low strength result from low tension and there may be rung variations between these two extremes. The inert gas is preferably argon, but the gas used and the pressure used depends on the properties desired. The heating is carried out in stages, preferably by passing a continuous filament through a series of ovens, but as a batch process the whole operation may be performed in one oven. If multiple ovens are used they may include a series of rolls which are independently driven so as to apply any desired tension in any stage of the process. Once the carbonization has progressed to the stage at which the filament becomes an electrical conductor or semi-conductor an electric current may be passed through it to obtain the desired heating. Means are provided for controlling this current, such as, using an alternator with controllable excitation so as to control the temperature generated in the filament by the passage of the current, so that the filament may be heated to a pre-set cycle without any recourse to expensive furnace equipment and so that very much more control of temperature of the filament in different stages of the process is possible. Moreover, very much less power is needed because the heat is applied only where it is wanted namely, in the filament. In general, it is desirable progressively to increase the temperature of the filament as crystallization proceeds and electrical heating as described is well adapted to providing these conditions.

In a modification, a yarn or braid or fabric may be wound on a beam such as a modified loom beam immediately after the initial heating process. This beam can be put into a pressure vessel such as a beam dye machine which is filled with the inert gas. Once heating has progressed to the point at which the yarn or braid or fabric is becoming conductive, the beam can be heated by means of induction using the normal technique or inducing eddy currents in the whole mass of yarns or braids or fabrics so as to cause them to heat up and promote crystallization of the carbon. Alternatively, an electric current can be passed through the beam or again high frequency heating as opposed to induction can be used in which case the whole heating sequence can be based on this method of heating.

The blowing of an inert gas through the yarn or braid or fabric may be assisted by the use of a vacuum to draw the gas through the beam at any desired pressure.

In the accompanying drawings:

FIG. 1 is a schematic diagram of an apparatus for carrying out the invention;

FIGS. 2 and 3 are diagrams illustrating modifications; and

FIG. 4 is an X-ray diffraction pattern.

The apparatus shown in FIG. 1 comprises a ceramic tube I wrapped with an electric heating coil 2. This tube acts as an oven. A portion of zinc (not shown) or other suitable metal is placed in the tube 1. The ceramic tube is lagged with metal reflecting foil (not shown) which may easily be removed to permit cooling of the tube during the latter parts of the treatment. Inside the tube 1 are two carbon or graphite electric contacts 3, 4 which are positioned so that a yarn F is forced to pass over each contact and make good electrical contact with them. The tube 1 is sealed, but has two openings 5 and 6, one at each end of the tube to allow an inert gas, preferably argon, to pass through. A thermocouple 7 is arranged in the tube to measure the temperature at any time.

A modified apparatus illustrated in FIG. 2 comprises a first heating chamber 10, a second heating chamber 11 communicating with the chamber 10, and a third chamber 12 communicating with the chamber 11. A Filament F is passed from a supply 13, through the three chambers and is received by a winder 14. The filament F travels through the chambers in a tortuous path, being passed over rollers 15. The lower rollers in the drawing are earthed (grounded) and an electric current is applied to the upper rollers. Thus, an electric potential will be maintained across the filament F. An inert gas is passed into the second chamber 11 through an inlet 16 and is withdrawn through an outlet 17 from the third chamber 12. The chamber is surrounded by heating means 18. The temperature is controlled by the amount of current travelling through the filament F.

In another modification illustrated in FIG. 3 the filament F is wound on a beam 20 contained in a pressure vessel 21. An inert gas is blown by a fan 22 into the interior of the beam 20. It passes into the vessel 21 through apertures 23 and is withdrawn from the vessel through an outlet 24. The vessel 21 is surrounded by heating means 25. The vessel is maintained under a vacuum by means 26. Intra-molecular heating is induced in the filament F by high frequency or radio frequency heating, by the passage of an electric current through the filament or by induction potential.

The invention is illustrated by the following examples:

EXAMPLE 1 Polyacrylonitrile tow was inserted in the ceramic tube 1 of FIG. 1. A weight of 170 gms. tensioned the tow. 15 gms. of metallic zinc were introduced into the tube and an inert atmosphere was maintained by passing argon through the tube.

The temperature of the tube was then raised to 600 C. over a period of 10 hours during which time the electrical resistance of the tow dropped from infinity to 0.5 ohm.

An of 48 volts was then applied to the yarn with a resistance of 5- /2 ohms in series so that a current of 8 amps flowed causing the yarn to heat up. This current was maintained for /2 hour and then increased to 22 amps in steps of approximately 2 amps over a period of 2 hours.

At this point the external resistance was removed and an EMF. of 3 volts was applied directly to the yarn giving a current of 40 amps. In a period of about 2 minutes, this was raised to amps at 7 /2 volts and maintained for about 1 second, after which the apparatus was allowed to cool.

It was found that the zinc had disappeared and the yarn had become black with considerable strength and flexibility. Subsequent examination of X-ray diffraction showed a strong peak at 13.2 giving a d value of 3.36 angstroms which spacing indicates complete graphitization. The yarn contained around 0.1% of zinc.

FIG. 4 of the accompanying drawing is the X-ray trace.

EXAMPLE 2 In this example, the apparatus illustrated in FIG. 1 was used to treat a dried tow of polyacrylonitrile. The polymer used was in a standard modified form. The denier of the sample was measured after drying the yarn in the dessicator.

The apparatus was assembled with the ends of the heating tube open so as to permit air to pass through by natural convection and the temperature was raised at 1 /2 C. per minute to 240 C. which was held for 15 hours. The filament was maintained under a tension of 0.01 gram/ denier to 0.5 gram/ denier. During this process the yarn changed color from white through orange to black but there was no obvious change in electrical conductivity.

15 g. of metallic zinc were introduced into the apparatus.

The apparatus was then sealed from the atmosphere and flooded with argon gas which was allowed to bleed through the system. The temperature was now raised at 1 C. per minute to 600 C. and the yarn resistance was continuously monitored by applying a small voltage across the yarn.

By experimentation it was found that monitoring the resistance by using voltages in excess of 40 volts caused the equilibrium of the yarn to be upset and better results were obtained by only attempting to raise the temperature of the yarn by electrical means after the yarn temperature had passed 600 C. Moreover, it was found during experimentation that after passing 240 C. rates of rise in temperature in excess of 1.5 C. per minute caused lower conductivities in the yarn at each stage in the experiment. It is believed that lower conductivities are associated with lower densities in the material and that lower densities are associated with lower strengths and modulus in the resulting yarns.

When the temperature of the apparatus had reached 615 C. and the yarn resistance had fallen to 1,660 ohms, an attempt was made to heat the yarn electrically by applying 240 volts across the yarn between the electric contacts. The was supplied by an alternator with a constant excitation current.

There was an immediate and sudden rise in the current from the expected monitored value of 144 milliamps. The voltage was reduced and when stability was achieved it was found that the yarn had ceased to show semiconducting properties and now had an electrical resistance of 7 ohms. The current of 10 amps which now flowed through the yarn was found to be enough to keep the temperature rising. The current was then slowly raised to 13.5 amps and the yarn could be observed to be heating the ceramic oven tube to white heat. This condition was maintained for 2 hours when the apparatus was allowed to cool.

It is believed that the initial polyacrylonitrile fiber passes through an intermediate pyridine-like ring structure which may be described as a polynaphthiridine ring structure. This is believed to be the origin of the exothermic reaction which has been observed. The cause is the change from a polymer chain to a more stable ring structure which is at a lower energy level and therefore causes the heat to be released.

The full scale deflection (F.S.D.) in FIG. 4, is measured in counts per second (C.P.S.). The two peaks in the right-hand half of FIG. 4 are the same as those in the left-hand half, but with a different full scale defection -(f.s.d.) being used. The horizontal axis is the axis along the fibre and the vertical axis is the axis across the fibre d is the intercrystal spacing in angstrom units (A.). The sharp numbered lines marked 12 and 16 are reference peaks to enable the exact location of the graph to be found. These numbers, together with the number 13.2 marked at the peak of the graph are the angles at which diffraction occurs and from which a substance may be identified.

I claim:

1. A method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heat treatment to graphitize said carbon-containing filaments in a furnace containing an atmosphere of an inert gas and a metal adsorbable by said carbon-containing filament and vaporizable at graphitizing temperatures, said metal being taken from the glOup consisting of the metals from Groups I, II, IV, and VI of the Periodic Table.

2. A method according to claim 1 comprising the steps of effecting said heat treatment by passing an electric current through said filament, and subjecting said filament to a voltage sufficient to cause the graphitization thereof.

3. A method according to claim 1 wherein said inert gas is argon.

4. A method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heat treatment to graphitize said carbon-containing filaments in a furnace containing an atmosphere of argon and zinc.

5. A method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heat treatment to graphitize said carbon-containing filaments in a furnace containing an atmosphere of an inert gas and zinc.

6. A method of making a carbon filament comprising the steps of subjecting a polyacrylonitrile filament to a heat treatment at a temperature of from 230 C. to 330 C. which produces a black color in the filament, providing the filament which has been preheated to said temperature of 230 C. to 330 C. with an enclosure containing an inert gas atmosphere and an electronegative metal with a boiling point below 3,000" O, subjecting said filament in said atmosphere and in the presence of said metal to a heat treatment at a temperature rising to about 600 C., whereby said filament becomes a resistance heater, and thereafter subjecting said filament to heat treatment in the graphitizing range of temperature by the passage of electric current therethrough, whereby said filament is graphitized.

7. A method according to claim 6 wherein said filament is held under tension while being subjected to heating.

8. A method according to claim 7 wherein said tension is from 0.01 gram per denier to 0.5 gram per denier.

9. A method according to claim 6 in which said temperature of about 240 C. is maintained for not less than two hours.

10. A method according to claim 6 in which said temperature is raised from 240 C. to 600 C. at a rate of from /2 to 1 /2 C. per minute 11. A method according to claim 6 in which the heating to eifect graphitization is performed by high frequency induction.

12. A method according to claim 6 in which the boiling point of the metal is below 2,000 C.

13. A method according to claim 6 in which the metal is selected from the group consisting of alkali metals and Group II metals of Mendeleefs Periodic Table of the elements.

14. A method according to claim 13 in which the metal is cadmium.

15. A method according to claim 6 in which the metal 1s zinc.

16. A method according to claim 6 in which the metal is calcium.

17. A method of making a carbon filament comprising the steps of subjecting a carbon-containing filament to heat treatment to graphitize said carbon-containing filaments in a furnace containing an atmosphere of an inert gas and a metal adsorbable by said carbon-containing filament and vaporizable at graphitizing temperatures, said metal being taken from the group consisting of Li, Na, K, Rb, Cs, Mg, Ba, Zn, Cd, Pb and Te.

18. A method according to claim 17 comprising the steps of effecting said heat treatment by passing an electric current through said filament, and subjecting said filament to a voltage sufiicient to cause the graphitization thereof.

19. A method according to claim 17 wherein said inert gas is argon.

References Cited UNITED STATES PATENTS 3,071,637 1/1963 Horn et a1 117--228 3,395,970 8/1968 Machell 117-46 X 3,547,677 112/ 1970 Gentilhomme et al. 11746 X RALPH S. KENDALL, Primary Examiner US. (:1. X.R.

117 22s, 227, 46 cc, 107.1 

